How to Play

What is Sudoku

Sudoku is a number placement puzzle popular with millions of players worldwide. Originally popularized
in Japan, the name Sudoku is a Japanese phrase meaning "single number." This refers to the puzzle's
gameplay where each square has a single possible number for any given puzzle. Each 9x9 grid has a unique
number combination resulting in a single valid solution.

How to Play

Each puzzle starts with an initial set of given numbers, colored in black. The objective of the game is to
find the numbers for the remaining squares. The only Sudoku rule is that each row, column, and 3x3 cube may
only have one instance of the numbers one through nine. Using the given numbers as hints, use this rule to
determine if a square must contain a certain number. As you fill in the squares, you gain new clues!

Basic Strategy

Check each row, column, and cube for what numbers are already revealed, in order to determine what number possibilities are
left for the remaining hidden squares. Cross check between row, column, and cube and you might find that a square only has
one possible answer. Alternatively, you might find that within one row, column, or cube that one number between 1-9 only is
possible in one square. That means you have solved that square.

For more advanced solving techniques, see the strategy section. The Easy puzzles on this site require only the most basic solving
strategies, with more complicated strategies required as the difficulty level increases.

Buttons

A number of tools are provided to help you solve puzzles. These are explained in order of appearance.

Error Check: This button will check if any row, column, or cube has multiple instances of the same number.

Edit Pencil Marks: This will mark or unmark a square with possible answers.

Autofill Pencil: Automatically fill all available squares with possible answers. You may use this only once per game
and is provided to avoid tediously filling out all squares with pencil marks.

Mouse only: Toggle filling out squares with the mouse or by entering numbers on the keyboard.

Pause: Stop the clock.

Show Solution: This reveals the solution, but also ends the game and prevents you from seeing the victory message
and from sharing your results.

About Sudoku Island

Sudoku Island

This website is dedicated to providing quality Sudoku puzzles online and free of charge. We hope you find
our puzzles challenging yet appropriately solvable, and that the site appearance is beautifully simple.

Software

Sudoku puzzles are generated on the server using proprietary Python code. A complete puzzled is generated, and the opening
squares are determined. The puzzle is then solved using different levels of strategy, and assigned a difficulty rating. The
puzzles are also solved by brute force in random order to reject puzzles that have multiple solutions.

For the front end, the technologies used are HTML5, jinja, CSS, Javascript, jQuery, and Bootstrap.